EPDM rubber, an elastic terpolymer of ethylene, an alpha-olefin such as propylene, and a diene such as ethylidene norbornene, has the molecular structure not including an unsaturated bond in the main chain, and has the characteristics of weather resistance, chemical resistance, and heat resistance that are superior to general conjugated diene rubbers. Due to the characteristics, the elastic terpolymer such as the EPDM rubber has been widely used as a material for the industrial materials such as for all sorts of automobile parts, electrical wire, construction and all sorts of hoses, gaskets, belts, bumpers, and a blend with plastics.
The elastic terpolymer such as the EPDM rubber has been mostly prepared by copolymerizing 3 kinds of monomers in the presence of a catalyst including a vanadium compound, for example, a vanadium-based Ziegler-Natta catalyst. However, since the vanadium-based catalyst has low catalytic activity, there is a need to use an excess amount of catalyst and a disadvantage that a residual metal content increases in the terpolymer. Therefore, processes for catalyst elimination and decolorization are required after the preparation of the terpolymer, and the residual catalyst in the resin may cause problems of heat resistance deterioration, impurity generation, hindrance to a vulcanization reaction, and so on. Further, it has not been easy to prepare the elastic terpolymer by using the catalyst including a vanadium compound due to its low polymerization activity and the polymerization condition of a low temperature, and it has been difficult to control the molecular structure of the copolymer because it has not been easy to control the uptake rate of comonomers such as propylene and diene. Therefore, in the case of using the vanadium-based catalyst, there has been a limit on the preparation of the elastic terpolymer with various properties. Recently, because of the problems, a method of preparing the elastic terpolymer, such as the EPDM rubber, by using a metallocene-based group 4 transition metal catalyst instead of the vanadium-based Ziegler-Natta catalyst, has been under development.
Such group 4 transition metal catalyst shows high polymerization activity in the polymerization of olefins, and makes it possible not only to prepare a copolymer having a higher molecular weight but also to easily control the molecular weight distribution and the composition of the copolymer. Furthermore, it has an advantage of enabling the copolymerization of various comonomers. For example, U.S. Pat. Nos. 5,229,478 and 6,545,088, and Korea Pat. No. 0,488,833 disclose that an elastic terpolymer having a high molecular weight can be obtained with excellent polymerization activity by using various metallocene-based group 4 transition metal catalysts obtained from the ligands such as cyclopentadienyl, indenyl, fluorenyl, and so on.
However, in the case of polymerizing 3 kinds of monomers by using existing group 4 transition metal catalysts, there has been a disadvantage that the repeating units derived from the monomers are unevenly distributed in the terpolymer chain due to their high reactivity to the comonomers of alpha-olefin. As a result, it has been difficult to obtain the elastic terpolymer such as the EPDM rubber having excellent elasticity and flexibility.
Further, U.S. Pat. No. 5,902,867 discloses the method of lowering the viscosity of the polymer by widening the molecular weight distribution for improving the mixing processability and the extrusion processability of the EPDM, but there is a problem that the polymer is degraded during the process by the low molecular components included in the cross-linked rubber product, and the surface characteristics and the low temperature characteristics decrease.
Therefore, it has been continuously required to develop an elastic terpolymer that can satisfy excellent processability, mechanical properties, and elasticity (flexibility) at the same time, and a preparation method that can prepare the elastic terpolymer with high productivity and yield rate.